Add USE_RBE support to soong.

Test: Built aosp_arm-user with and without USE_RBE. USE_RBE uses
a proxy script in place of rewrapper.

Change-Id: I5bf008a940513872d70b5b215bd6209f759826ae

ui/build/build.go

@@ -161,6 +161,11 @@ func Build(ctx Context, config Config, what int) {
 		startGoma(ctx, config)
 	}
 
+	if config.StartRBE() {
+		// Ensure RBE proxy is started
+		startRBE(ctx, config)
+	}
+
 	if what&BuildProductConfig != 0 {
 		// Run make for product config
 		runMakeProductConfig(ctx, config)